New Phoma Disease of the Swede. 



453 



-breaking down the tissues of the host. The mycelium 

 spread superficially, and as the hyphse attacked the living 

 cells, these lost their turgidity, the cell-sap evaporated, and, 

 as a result, the cells dried up, causing the depressions, and 

 finally the cracks, to appear. The change of colour was 

 also well marked, the purple colour held in solution was 

 destroyed (probably by an acid secretion of the fungus), 

 unmasking the green of the chlorophyll corpuscles, which, 

 later on, was itself destroyed, and only the colourless dead 

 cells remained within the area ot the spreading hyphae. In 

 a short time the pycnidia were developed, being at first 

 embedded in the superficial tissues ; but, later on, as they 

 reached maturity, they emerged, and the dehiscence ot 

 the spores took place. The cycle was thus completed 

 from spore to spore in the cultivations on the swede. 

 Artificial attacks of Phoma were repeatedly produced in this 

 manner, and afford convincing proof that the disease is due 

 to this fungus, and that it is spread by means of the 

 spores. 



Phoma occurring in the pycnidial stage is probably only 

 -a part of the life cycle of some other fungus, but if this 

 is so and what fungi [complete the cycle is at present 

 uncertain. Prillieux* and Delacroix consider that Phoma 

 Bctcs is a stage in the life cycle of Sfihcerella tabifica, an 

 ascigerous form found on the dead petioles of the beet. 

 These authors, however, admit that this has not been incon- 

 testably proved by actual cultures. Hitherto I have been 

 ^unable to find any Phoma attack on the leaves of the 

 •swede, or any ascigerous stage. 



The genus Phoma is accredited with a large number of 

 species, Saccardof enumerates as many as 638. Franks 

 gives 44, which are parasitic on various host plants, three of 

 which attack the cruciferous genus Brassica. I believe the 

 Phoma to be more generally destructive to cultivated crops 

 in Great Britain than is commonly recognised. Phoma Beta 



" Prillieux, Maladies des Plantes Agricoles, Tome II., 1897. 



f Saccardo, Sylloge Fungorum,Vol. Ill, 1884. 



X Frank, Die Pilzparasitaren Krankheiten der Pflanzen, 1S96. 



